Conventionally, many conventional synthetic resin caps have been used provided a top plate portion and a cylinder portion hanging from the circumferential edge thereof, with an annular inside seal projection to be fitted into the container mouth portion protrudingly formed on the inner surface of the top plate portion (refer for example to Japanese Unexamined Patent Application No. 2002-211605).
FIG. 6A and FIG. 6B show an example of a synthetic resin cap having an inside seal projection, the cap 31 including a cap body 4 having a top plate portion 2 and a cylinder portion 3 hanging from a circumferential edge portion 2b thereof.
The horizontal score 6 demarcates the cylinder portion 3 into a main portion 8 and a tamper evidence ring portion (TE ring portion) 9 connected to the main portion 8 by bridges 7.
On the inner surface of the main portion 8, a thread portion 10 is formed for threaded engagement with a male screw 22 formed on an outer surface 21c of a container mouth portion 21.
On the inner surface of the TE ring portion 9 are provided tabs 11 that block movement of the TE ring portion 9 by locking to the container mouth portion when opening the cap 1.
On the inner surface 2a of the top plate portion 2 is formed an annular inside seal projection 12 to be fitted into the container mouth portion 21. On the outer surface of the tip portion of the inside seal projection 12 is formed an annular abutting convex portion 12a to abut the container inner surface 21a. 
On the top plate portion 2 are formed an opening end seal projection 13 to abut an opening end face 21b of a container mouth portion 21 and an outside seal projection 14 to abut the outer surface 21c of the container mouth portion 21.